The presence in human and murine cancers of tumor infiltrating CD8+ T cells ('TIL') that express hallmarks of activation and maturation [cell surface markers, effector phase cytokine mRNAs, and perforin/granzyme-loaded lytic granules] demonstrates that tumors express antigens capable of eliciting immune response. Systemic immune response is intact in cancer patients and animal models, however, TIL have no lytic function in situ (or in vitro) thus demonstrating tumor-induced immune suppression that is likely to underlie tumor escape from elimination by antitumor immune response. Defective TIL lytic function is the result of tumor-induced blockade of T Cell Receptor (TCR) signal transduction, which our lab has previously shown to result from functional inhibition of p56lck kinase. Inhibition of TIL p56lck activity and defective lytic function is reversible both being recovered upon purification implying a tumor-derived, contact-dependent, fast-acting biochemical inhibitory switch. We have recently identified in nonlytic TIL a novel interacting partner for p56lck: Protocadherin-18 ('pcdh18'). Pcdh18 is a cell surface adhesion molecule expressed in various tissues, however, in the hematopoietic system, is expressed only in memory CD8+ T cells. We have recently determined that pcdh18 is an immediate-early gene transcribed upon activation of CD8+ central memory T cells (CD44+CD62LhiCD127+) of any origin (arising endogenously or induced by deliberate viral or bacterial infection, tumor growth, or transfer of allogeneic cells), coincident with conversion into effector-memory cells (CD44+CD62LloCD127+). We have shown that TIL are effector-memory cells. Importantly, we have recently determined that transfection of pcdh18 into primary CD8+ T cells causes defective: proximal TCR signaling, cytokine secretion, and lytic function, and enhanced activation-induced cell death, a phenotype that mimics that of freshly-isolated nonlytic TIL. Thus, we have identified an activation marker of CD8+ central memory T cells which functions as a negative regulator of proximal TCR signaling and effector phase function. Interestingly, pcdh18 contains a tyrosine motif at residue 842 (QGQYQP), which is highly homologous to the inhibitory tyrosine motif found in p56lck at tyrosine 505 (EGQYQP). Pcdh18 Y842 is robustly phosphorylated in nonlytic TIL but is not in lytic TIL suggesting an association between Y842 phosphorylation and defective TIL TCR-mediated signaling, and thus lytic function. Utilizing site-directed mutagenesis, I have shown that Y842 is required for the pcdh18 inhibitory phenotype in transfected T cells. p56lck Y505 is phosphorylated by C-Src tyrosine kinase ('Csk'), a kinase that inhibits T cell activation. Because of its high degree of homology with the p56lck inhibitory motif at Y505, I hypothesize that phosphorylation of pcdh18 Y842 by Csk regulates its ability to inhibit p56lck function. Since activation of Csk is regulated through the Protein Kinase A pathway, definitive assignment of its role in activation of pcdh18 will guide analysis of extracellular signals generated in the tumor microenvironment that induce lytic dysfunction in TIL.